[Wien] spin-orbit (PBE and mBJ) for perovskites

Mon Dec 9 21:32:06 CET 2019

Dear Prof. Blaha,
Thank you for clarification. I think, now I more or less understand what is happening. 
It seems, this issue is very easy to miss. Do you have any practical recommendation how to spot this kind of problem in the calculations of non-trivial material with few atoms?

Sincerely yours,
Mikhail Nestoklon

>Понедельник, 9 декабря 2019, 19:02 +03:00 от Peter Blaha <pblaha at theochem.tuwien.ac.at>:
> 
>Hi,
>
>Fist of all: The RLOs (p-1/2)-LOs were originally designed to improve
>the SO splitting of semicore states. There it makes a big difference and
>is essential to converge E-tot in a meaningful way.
>
>(By meaningful I do not mean the absolute number as you tested it, but
>an E-tot difference of 2 calculations at eg. 2 volumes).
>
>This can be best seen if you compare these semicore eigenvalues with the
>eigenvalues of case.outputst, where the fully-relativistic splitting can
>be found and compared.
>
>You can also add it in a case like Pb (usually, Pb does not have
>semicore p-states), but the effect MUST be small.
>
>Most importantly, you MUST NOT have a too large E-max, because when the
>spurious LO-eigenvalues of the APW+lo method enter in the spectrum at
>high energy, you may get a problem as indicated below:
>
>I checked this for a free Pb-atom (in a big box).
>
>lapwso with Emax=10 gives:
>
>           -1.5996189 -1.5996189 -1.5996189 -1.5996189 -1.4111046
>           -1.4111046 -1.4111046 -1.4111046 -1.4110328 -1.4110328
>           -0.8489490 -0.8489490 -0.3028265 -0.3028265 -0.2040009
>           -0.2040009 -0.2040009 -0.2040009 -0.0106114 -0.0106114
>
>while with EMAX=15 I get 6 spurious ghostbands ("bad Pb-5p states):
>           -8.2276000 -8.2276000 -3.1336676 -3.1336676 -3.1336591
>           -3.1336591 -1.6001880 -1.6001880 -1.6001880 -1.6001880
>           -1.4113465 -1.4113465 -1.4113464 -1.4113464 -1.4112735
>           -1.4112735 -0.8489490 -0.8489490 -0.2886641 -0.2886641
>           -0.2000960 -0.2000960 -0.2000960 -0.2000960 -0.0106114
>
>The SO-splitting of 6p states in the Pb atom is 0.109 Ry, while the
>"good" lapwso calculations yield 0.099, but the "bad ones only 0.088
>Without the RLO the SO splitting is 0.092, so still better than the
>"bad" calculation.
>
>Eventually one can avoid this switching back to LAPW for the Pb-p states.
>
>Peter Blaha
>
>On 12/9/19 3:30 PM, Mikhail Nestoklon wrote:
>> Dear Dr. Tran,
>> Thank you for the suggestion. Indeed, for CsPbCl3 I get very similar
>> values (0.70 for PBE-SO and 1.67eV for TB-mBJ) and if I add RLO on Pb
>> for CsPbI3 at least for PBE-SO I have something close to value given in
>> Jishi.
>> However, now I have a general question. How to understand that I need to
>> add RLO in this situation? Should I just try to add RLO on heavy atoms
>> and if the result changes prefer the values given by +RLO calculations?
>> Or I had to suspect that I need to add RLO when realized that the result
>> does not converge until de>15?
>> Sincerely yours,
>> Mikhail Nestoklon
>>
>> Пятница, 6 декабря 2019, 0:39 +03:00 от "Tran, Fabien"
>> < fabien.tran at tuwien.ac.at </compose?To=fabien.tran at tuwien.ac.at>>:
>>
>> Hi,
>>
>> Some time ago I did calculations on CsPbCl3 and I could reproduce
>> Jishi's results reasonably well except the one in the 1st
>> column("GGA") which is probably wrong. I got 0.70 eV (0.71 eV from
>> Jishi) for "GGA+SOC", 1.68 eV (1.59 eV from Jishi) for TB-mBJ and
>> 2.86 eV (2.83 eV from Jishi) for "present". The only special
>> requirement that was needed is a p1/2 LO on Pb. I used RKMAX=9 and
>> de=10 in case.in1. The bug reported here
>>
>>  http://zeus.theochem.tuwien.ac.at/pipermail/wien/2019-November/029882.html
>>
>> is active but has very small effect of 0.01 eV for GGA+SOC. Try
>> CsPbCl3 instead (struct and inso are attached).
>>
>> FT
>>
>> ------------------------------------------------------------------------
>> *From:* Wien < wien-bounces at zeus.theochem.tuwien.ac.at > on behalf of
>> Mikhail Nestoklon < nestoklon at mail.ru >
>> *Sent:* Thursday, December 5, 2019 8:21 PM
>> *To:* A Mailing list for WIEN2k users
>> *Subject:* [Wien] spin-orbit (PBE and mBJ) for perovskites
>> Dear wien2k community,
>> I plan to do some DFT calculation of inorganic perovskites using
>> WIEN2k (19.1 with some patches except the last one for RLO).
>> I’ve started from attempt to reproduce the values from Jishi et al.,
>> JPCC 118, 28344 (2014), but can not get the numbers given in Table 2
>> even for cubic CsPbI_3. The difference seem to stem from the spin-orbit.
>> What I did:
>> Using the file in attachment (I use the lattice constant given in
>> table 1 and R_MT indicated in the text) I do
>> $ init_lapw -b -vxc 13 -rkmax 9.0 -lvns 6 -fftfac 4 && x kgen
>> (14x14x14) && run_lapw -ec 0.00001 -cc 0.0001
>> I use -lvns 6, the difference with the result using standard value
>> is small, in Jishi et al. it is mentioned that l_{max}=10 which is
>> default, assuming they might have meant lvns I tried lvns=10, the
>> difference with lvns=6 is close to zero. I also tried to put Pb p3/2
>> orbitals to core (if I put P1/2 to valence the result is strange,
>> but this is a separate question) and see no difference.
>> For k-mesh I check the convergence and see that for 14x14x14 the
>> total energy convergence is below 1mRy and GAP is below 1 meV which
>> is fine, so I am using this k mesh.
>> Then I check the RKMAX convergence. For Rmt*Kmax =9 without
>> spin-orbit I get the same number as in Table 2, but I see that this
>> number is not fully converged: if I increase RKMAX further the total
>> energy decreases for 8mRy and gap increases for almost 6 meV. I find
>> it acceptable to use RKMAX=12: only then it is <1mRy and ~1meV for
>> total energy and gap respectively. The gap with these numbers is
>> 1.329meV which is 5 meV different from Table 2, this difference is
>> probably acceptable.
>> However, when I switch on the spin-orbit, the difference is huge.
>> With the default values (I only increase llmax, however it does not
>> change much) I get GAP about 0.269meV, which is almost 4 times
>> different from the value given in Jishi et al. Table 2. As the SO
>> value depends on de (in case.in1), I increase this parameter and
>> check the value of GAP and also spin-orbit splitting of conduction
>> and valence band (difference between energies of Gamma_8 and Gamma_7
>> in R point). GAP and spin-orbit in conduction band fully (below
>> 1meV) converge only when de=15. Still, the band gap is 0.259eV which
>> is too far from value given in Jishi et al.
>> The RLO (I tried to add it on Cs, as for Pb it should be useless
>> [1]) does not help
>> With the same parameters I do the mBJ run (TB-mBJ). As expected, the
>> gap increases, but up to 0.722 eV which is two times different from
>> the value given in Jishi et al.
>> I did not try to change R_MT: I use the value given in their paper.
>> The fact that I do reproduce the number given for PBE without
>> spin-orbit indicates that I hardly did any mistake in the structure.
>> However, the difference in the numbers with spin-orbit is too large
>> to be explained by unconverged results, both on my and their side.
>> Could you help me to understand, how can I reproduce their results?
>> Thank you in advance.
>> Sincerely yours,
>> Mikhail Nestoklon
>> [1]
>>  https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg17828.html
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>
>--
>
>                                       P.Blaha
>--------------------------------------------------------------------------
>Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
>Phone:  +43-1-58801-165300 FAX:  +43-1-58801-165982
>Email:  blaha at theochem.tuwien.ac.at WIEN2k:  http://www.wien2k.at
>WWW:  http://www.imc.tuwien.ac.at/TC_Blaha
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